The present invention relates to a pneumatic controller in a printing machine for detection of sheets held on a sheet feed cylinder and, more particularly, to a pneumatic controller which acts to idle or halt the printing machine, when sheets are not present on the sheet guide cylinder.
This type of device is used particularly in printing machines, in which the sheet for the first form and second working is turned according to the principle of sheet rear edge turning for control, if the sheet is on the sheet feed cylinder.
According to DD-PS 212 024 the control occurs by sensing and transmitting devices attached to opposing printing machine walls. The sensing and transmitting devices can communicate using ordinary light, laser light or ultrasonic waves. The detecting beam can be inclined at from 10 to 40 degrees to the cylinder axis. The sheet feed runs, when the detection beam extending from the transmitting device passes through the correctly fed sheet in the inclined direction and continues with reduced intensity to the sensing device, so that no signal is generated in the sensing device and subsequent commands in connected devices are not generated. When a sheet is not properly in place during the sheet feed operations and the detection beam does not pass through the sheet, the full intensity of the unattenuated detection beam impinges on the sensing device and the sensing device generates a output signal activating additional commands, such as printing machine halt and reduction of machine speed or a machine halt, in controlling mechanisms.
A disadvantage of the controller described in the above paragraph is that its performance depends on the paper characteristics such as its transparency, surface properties and/or its weight and this requires time-consuming adjustments. An additional disadvantage is that the controller is only reliable up to a certain minimum size. With smaller sizes the desired switching off of the sensing device may no longer occur and the controller functions incorrectly. An additional disadvantage is the high sensitivity of the device to dirt and dust.
A sheet responsive device is also described in DE-AS 26 21 289, in which the sheet located on the sheet turning drum is detected by a row of suction devices at its rear edge. With only partial covering of the suction devices arranged in a row in the device, the flow of air is interrupted and a provided pressure gauge and connected controlling mechanisms cause the idling or the halt of the printing machine.
The controller described in the paragraph immediately above has the disadvantage that the control depends on the measurement of the low pressure in the air flow at the suction devices located on the sheet turning device. With optimum design of the entire pneumatic system for control of the suction devices, the low pressure in the suction air guides or ducts, changes only slightly during the air flow through them and the control is very sluggish. Since, above all with a high sheet feed rate only a short time is available for sheet control, this device does can make errors, particularly at high sheet feed rates.